Drive shaft coupling

ABSTRACT

Disclosed is a torque-transmitting coupling assembly and method, as well as flexible surgical reamers having such assemblies. The reamers employ a fitting with radially flexible member, e.g., a split collet or a super-elastic collar that receives an elongated tubular shaft of super-elastic alloy. A compression sleeve is applied to the collet, in one embodiment, while the collar is located within a counter-bore of the fitting in another embodiment. Relative motion between the shaft, fitting and collet (or collar) induces a super-elastic activation in the shaft to form the desired coupling.

RELATED APPLICATIONS

This application is a continuation-in-part of Ser. No. 09/860,918, filedMay 18, 2001 and entitled, “Stress-Induced Connecting Assembly”, (nowU.S. Pat. No. 6,513,814), which is a continuation-in-part of Ser. No.09/523,719, filed Mar. 11, 2000 and entitled, “Stress-Induced InterposedConnector” (U.S. Pat. No. 6,257,953), which is a continuation-in-part ofSer. No. 09/311,938, filed May 14, 1999 and entitled “Stress-InducedSeal”, now abandoned. This application is further based on priorProvisional Application Ser. No. 60/262,362, filed Jan. 19, 2001 andentitled Drive Shaft Coupling. The entire disclosures of theseafore-mentioned applications are expressly incorporated by referenceherein and relied-upon.

TECHNICAL FIELD

This invention relates generally to torque-transmitting assemblies thathave fittings coupled to a flexible drive shaft made of a super-elasticalloy, useful in devices for medical and industrial applications where aflexible shaft is necessary; and in particular to powered surgicalinstruments for transmitting torque to “flexible reamers” or “flexibledrills” to remove material from the center of curved bones duringorthopedic surgery.

BACKGROUND OF THE INVENTION

The use of metallic super-elastic alloys, such as Ni—Ti (nitinol) andother bi- or tri-metal alloys, has been documented in a variety oftechnical applications, including fasteners, connectors, gaskets, clampsand seals. Many such uses have required temperature in order to activatethe material and change its physical state, while others have usedmechanical forces that impart stress to cause a super-elastic physicaldeformation in the material. Of particular concern to the instantinventor is the application of the super-elastic material to connectors.The use of non-corrosive, metallic super-elastic material offers adecided advantage in high performance connecting assemblies, versus moreconventional connectors requiring threaded fasteners, springs, clamps orother holding or securing mechanisms. Particularly it can withstand morewear than alloys used in conventional connectors due to its hardersurface characteristics. It can also withstand extreme vibrations andnot loosen due its elastic preloaded condition without usingconventional adhesives to hold the assembled components and/or theconnector itself together. Adhesives used with conventional connectorsmake them very difficult to disassemble, whereas it is generallypossible to make a super-elastic connector completely reversible.

U.S. Pat. Nos. 5,395,193 and 5,584,631 to Krumme et al., discuss the useof nickel-titanium shape memory retainers in an optimized elasticcondition that have super-elastic or pseudo-elastic properties. Thesefasteners are said to be useful for eyeglass assembly; they are placedonto a pin to retain components together.

It is commonly known that nitinol (formally known as Nickel TitaniumNaval Ordinance Laboratory, but other super-elastic nickel-titaniumalloys being included in this definition) tubing, wire or rod can beused as a mechanical drive shaft. The use of metallic super-elasticalloys, such as Ni—Ti (nitinol) and other bi- or tri-metal alloys, hasbeen documented in a variety of technical applications, includingfasteners, connectors, gaskets, clamps and seals. Many such uses haverequired temperature in order to activate the material and change itsphysical state, while others have used mechanical forces that impartstress to cause a super-elastic physical deformation in the material. Ofparticular concern to the instant inventor is the application of thesuper-elastic material to connectors. The use of non-corrosive, metallicsuper-elastic material offers a decided advantage in high performanceconnecting assemblies, versus more conventional connectors requiringthreaded fasteners, springs, clamps or other holding or securingmechanisms. Particularly it can withstand more wear than alloys used inconventional connectors due to its harder surface characteristics. Itcan also withstand extreme vibrations and not loosen due its elasticpre-loaded condition without using conventional adhesives to hold theassembled components and/or the connector itself together. Adhesivesused with conventional connectors make them very difficult todisassemble, whereas it is generally possible to make a super-elasticconnector completely reversible.

U.S. Pat. No. 5,683,404 to Johnson, entitled “Clamp and Method for itsUse”, further discusses shape memory materials that are“pseudo-elastic”, defining these materials to be super-elastic, becauseof their ability to exhibit super-elastic/pseudo-elastic recoverycharacteristics at room temperature. In other words, a material issuper-elastic when, if sufficient stresses are applied, such materialsexhibit martensitic activation (i.e., deform from an austenitic crystalstructure to a stress-induced structure postulated to be martensitic innature), returning thence to the austenitic state when the stress isremoved. The alternate crystal structures described give the alloysuper-elastic or pseudo-elastic properties. Poisson's Ratio for nitinolis about 0.3, but this ratio significantly increases up to approximately0.5 or more when the shape memory alloy is stretched beyond its initialelastic limit it is at this point that stress-induced martensite is saidto occur. i.e., the point beyond which the material is permanentlydeformed and thus incapable of returning to its initial austeniticshape. A special tool is employed by Johnson to impart an externalstretching force that deforms the material which force is then releasedto cause the material to return to its original condition. While thedevice is stretched, a member is captured by it and securely clampedwhen the stretching force is released. This device is intended for usein clamping and does not contemplate traditional connecting operationsof the kind addressed by the present invention. Another use envisionedby Johnson is in connecting the modular components of a medical device,as described in his U.S. Pat. No. 5,858,020, by subjecting a thimblecomponent made of shape memory material to an external stretchingstimulus to elongate and thereby reduce its transverse dimension. Uponrelease of the stretching force, this component returns towards itsoriginal rest dimension, contacting and imparting a force on anothercomponent. This is a sequential stretching and relaxation of thesuper-elastic material rather than a simultaneous activation andretention operation. Also, special structures are necessary on thethimble to allow the stretching force to be imparted.

In U.S. Pat. No. 5,197,720 to Renz, et al., a work piece is held withina clamping tool by an expansion element made of shape memory materialthat is activated by mechanical force. In this way, torque istransmitted through the shape memory member. This device is useful forbringing parts together for holding the work piece in order to performan operation. It does not, however contemplate a use as a connector.U.S. Pat. No. 5,190,546 to Jervis discloses insertion into a broken bonecavity of a split member made of shape memory material using aSUPER-elastic alloy. The split member holds the walls of the bone cavitywhen radial compressive forces acting on it are released. In order forthe radial compressive force to reduce the diameter, the component mustbe split, allowing the reduction in dimension for insertion. It does notact as an interposed member in a connecting assembly.

Others have sought to utilize the properties of shape memory materialsas locking, connector and bearing elements, e.g. U.S. Pat. No. 5,507,826to Besselink et al, U.S. Pat. No. 5,779,281 to Kapgan, et al., and U.S.Pat. No. 5,067,827 to Arnold, respectively; however, such approacheshave required temperature to be applied during use. U.S. Pat. No.5,277,435 to Kramer, et al. and U.S. Pat. No. 5,876,434 to Flomenblit,et al. similarly has relied upon temperature to activate the shapememory effect. Such dependence on extrinsic activation by temperatureintroduces an added process step and may further be disadvantageous incertain other applications.

U.S. Pat. No. 5.842.312 to Krumme, et al., entitled, “Hysteretic DampingApparati and Methods”, employs shape memory tension elements to provideenergy dissipation. Such elements can be placed between buildingstructures, etc. which are subject to vibration, serving to absorb theenergy created by their relative movement. However, this patent does notcontemplate the vibration dampening effect of a SUPER-elastic materialin the formation of a connector.

Nitinol is especially useful for transmitting torque while in a bowed orbent shape. These types of drive shafts have proven useful in orthopedicsurgical applications where drilling or reaming of curved bones isnecessary. One application is to use a drill or reamer with a nitinoldrive shaft to clean out the center of a femoral bone before implantinga prosthesis or femoral nail. These bones typically have a bow with a90-inch radius and require a flexible reamer for the procedure. Nitinoltubing can be used for this application since it is cannulated and canbe passed over a guide wire that is placed down the femur before thereaming process begins. Since the tubing is solid it is very easy toclean after the surgical operation since there are no crevices for bloodto get trapped in. Earlier designs utilized spring drive shafts andcleaning was extremely difficult since blood could get trapped betweenthe windings of the spring. The earlier spring designs also haddifficulties when run in the reverse direction since springs tend to bestrong while being used in one direction, however when run in theopposite direction they tend to unwind. To prevent this unwindingproblem several manufacturers have added an additional spring inside ofthe primary spring, which is wound in the opposite direction. Since onespring is inside of the other this contributes to the difficulties withcleaning and further obviates the need for an alternative shaft design.

With the market demand increasing for these novel nitinol drive shaftsthere have been many attempts to develop safe coupling methods forattachment to the shaft. One difficulty that engineers have been facedwith is presented when nitinol tubing exceeds its torsional or fatiguestress limits; it has been known to fail catastrophically and fragmentinto several sharp pieces. This is dangerous when inside of a patientand poses severe concerns if these types of products are to be usedreliably. Historically there have been no solutions offered to limit thestress in the drive shaft, which would eliminate the presently lingeringconcerns over breakage during use.

Another difficulty is presented with the attachment of the fittings tothe nitinol drive shaft. The connection must be reliable and not createany unnecessary stress on the tubing. This will lead to early failure ofthe shaft. Typically in the orthopedic reamer example mentioned aboveone end of the nitinol tube has a stainless steel fitting which attachesto a power instrument and on the opposite end either a stainless steelreamer head or an intermediate modular fitting that connects to a reamerhead. Several attempts to create reliable attachments have been made.

One approach to the above-mentioned attachment problem has been to usean epoxy to “glue” the fittings onto the nitinol shaft. However,temperatures in the sterilization process and the criticality of surfacepreparation have rendered this approach unreliable.

Another approach has been to attach the fitting to the shaft with alaser weld; however, the welding process embrittled the tubing and itwas known to fail torsional demands in testing. A cross hole and pinwere placed through the fittings, however this added approach furtherproved useless since the matching hole in the tubing created atremendous stress riser in the tubing causing failure at very lowtorsional values. In the example mentioned it was known to fail anywherebetween 2 to 4 N–M.

Yet another approach has been to press fit the nitinol shaft into afitting with approximately 0.002-inch interference. Initial trialsworked, however when put through rigorous fatigue tests the tubingplaced too much hoop stress on the fittings causing them to fail ratherthan the shaft. The solution proposed to fix that problem was to add along section on the fitting that was loosely fit around the tube. Thiswould allow the stress to transition slowly into the area where thepress fit was done. This worked successfully, however the solutioncreated a need for the fitting to be extremely long in comparison to thereamer heads being used. This is undesirable since the reamer mustfollow the curvature of the bone and it did not solve the issue oflimiting the torque in the shaft to ensure the safety of the drive shaftduring use.

Thus, there remains a primary need to provide a coupling system that issafe and effective for use in surgical and industrial applications whereflexible drive shafts are necessary.

Accordingly, there is a need to form a connecting assembly using adurable metallic, non-corrosive connector assembly, which are simple toinstall using relative motion to activate the assembly.

There is a further need to form a secure connection between componentsthat minimizes the micro-motional wear characteristics of the assembly,enhancing its useful life.

There is another need to form a fastened assembly that does not requiretemperature for its activation.

There is still a need to form an assembly using a fastener that adjustsfor differences in thermal coefficients of expansion or contraction ofdissimilar materials comprising those components being fastened.

There is still a further need for a connector with elastic propertiesthat allow more forgiving tolerances during manufacturing of theassembly components.

There is another need to use a nitinol drive shaft to replace the springdrive shafts in orthopedic instruments and many industrial tools tosimplify the cleaning process and ensure consistent torque resistance inthe forward and reverse directions.

There is a further need to provide a coupling system which will limitthe torque in the nitinol drive shaft fitting to ensure that thecoupling limits the torque before the tubing breaks.

There is yet a need for a coupling that will not place any unnecessarystress on the nitinol tubing causing it to prematurely break.

There is also a need to shorten the length of the fitting so that thereamer can follow the natural contour of the inside of the bone whiletransferring the stress smoothly so as to ensure the strength of thefitting.

SUMMARY OF THE INVENTION

According to the invention, a torque-transmitting assembly is described,as well as a method of forming the assembly and a surgical reamer thatincludes the assembly. The assembly has a female coupling member with abore. The female coupling member may be a fitting that connects to apower instrument or it may present a cutting head, or both. A radiallyflexible member is received within the bore, defining a hollow shapewith an opening. The assembly also has an elongated shaft member made ofa super-elastic alloy, received within the opening. Relative motionamong the members causes the radially flexible member to contact theshaft, inducing a super-elastic activation in the shaft that urges theshaft and radially flexible member into surface-to-surface contact,securing the members together in a fixed relative position. In apreferred embodiment, the radially flexible member has an externalsurface that frictionally engages the bore upon relative motion. Thecontact still preferably occurs along one or more areas thatfrictionally carry the applied torque, which contact area may becalibrated so that the contact slips at a preset torque before thefailure strength of the shaft is reached. In another preferredembodiment, the shaft is tubular with a cannulation, which may furtherbe aligned with another cannulation in the female coupling member forcommon passage of a guide wire. In yet another preferred embodiment, aninter-positional polymer sleeve is provided in the assembly fortransmitting bending stress. In a still another preferred embodiment,the female coupling member has a counter-bore, while the radiallyflexible member has an exterior surface adapted for engagement withinthe counter-bore and may be compressed within the counter-bore or be ina pre-assembled state therein. In one alternative, preferred embodiment,the radially flexible member has a split collet, whereupon relativemotion among at least two of the members causes the opening to contactthe shaft, inducing a super-elastic activation in the shaft that urgesthe shaft and the collet into surface-to-surface contact, securing themembers together in a fixed relative position. In another alternative,preferred embodiment, the radially flexible member is a collar and madeof super-elastic alloy, whereupon relative motion between the fittingand the collar causes the collar to contact the shaft, inducing asuper-elastic activation in the shaft that engages the shaft and collarinto surface-to-surface contact, securing the members together in afixed relative position. The collar may further be a washer or a seriesof washers.

A method of forming a torque-transmitting assembly is also disclosed,having the following steps. A female coupling member is provided, with abore, as is a radially flexible member with an external surface and anopening, the radially flexible member being situated within the bore. Anelongated shaft member is provided, made of a super-elastic alloy, andis received within the opening. Relatively moving at least two of themembers, causes the radially flexible member to contact the shaft,inducing a super-elastic activation in the shaft that urges the shaftand radially flexible member into surface-to-surface contact, securingthe members together in a fixed relative position. The radially flexiblemember may alternatively consist of either a split collet operable withan outer compression sleeve, or a super-elastic collar operable within acounter-bore of the female coupling.

A flexible surgical reamer having a torque-transmitting assembly is alsodisclosed. The assembly includes a female coupling member, which may bea fitting with a counter-bore that is further provided with a headpresenting a cutting tool-bit, or may be an outer compression sleeve. Aradially flexible member is provided, which may be a collar member madeof super-elastic alloy with an opening, preferably a washer or series ofwashers, located in the counter-bore. Relative motion among the memberscauses the opening to contact the shaft, inducing a super-elasticactivation in the shaft that urges the shaft and the collar intosurface-to-surface contact, securing the members together in a fixedrelative position. Alternatively, the radially flexible member may havea split collet with an exterior surface received in the sleeve, and anopening. The assembly has an elongated shaft member made of asuper-elastic alloy, received within the opening. Relative motion amongthe members causes the opening to contact the shaft, inducing asuper-elastic activation in the shaft that urges the shaft and thecollet into surface-to-surface contact, securing the members together ina fixed relative position. It is desirable that the contact occurs alongan area that frictionally carries the applied torque and is calibratedto slip at a preset torque before the failure strength of the shaft isreached. An inter-positional polymer sleeve may be additionally providedfor transmitting bending stress in the assembly. The shaft may betubular, with a cannulation, likewise the female coupling member mayhave a cannulation that aligns with the shaft cannulation for passage ofa guide wire through the reamer.

An advantage of the present invention is a torque-transmitting couplingassembly that is also torque-limiting, relying upon a surface-to-surfacefrictional contact, thus slippage of the coupling occurs at torque lessthan the maximum failure strength of the shaft.

Another advantage of the present invention is that the torque can beadjusted, by increasing or decreasing the area of contact ininterference and adjusting the surface finishes on the assemblycomponents.

A further advantage is the amelioration of fatigue due to hoop stress byuse of a preferred softer material, such a polymer sleeve interposedbetween the coupling member and the shaft—this acts as a dampener tospread out the forces smoothly ensuring the proper stress transfer.

Other objects and advantages of the invention will become apparent uponreading the following Detailed Description and upon reference to theappended Drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an exploded view of a torque-transmitting assembly of thepresent invention, showing a preferred fitting formed with a splitcollet, a compression sleeve and an optional polymer sleeve interposedtherebetween;

FIG. 2 is an elevational view of FIG. 1, showing the collet compressedby the sleeve to couple the assembly;

FIG. 3 is a sectional view taken substantially along the Lines 3—3 ofFIG. 2;

FIG. 4 is an exploded view of another embodiment of atorque-transmitting assembly of the present invention, showing a fittingwith a collar preferably having a series of washers each made ofsuper-elastic alloy, a compression sleeve and an optional polymersleeve;

FIG. 5 is an elevational view of FIG. 4, showing the collar compressedby the sleeve to couple the assembly; and

FIG. 6 is sectional view taken substantially along the Lines 6—6 of FIG.5.

DETAILED DESCRIPTION

Referring to FIGS. 1–3 and particularly FIG. 1, the present inventionincludes a drive shaft 5 made of a super-elastic alloy, preferably anickel-titanium commonly known as nitinol. Shafts made from this type ofalloy can be formed with a cannulation 7 as shown in FIG. 3 and exhibitthe distinctive characteristic of transferring torque while subjected tohigh bending forces during use. The exemplary use of the presentinvention is in orthopedic drilling and reaming devices; however, theusage of super-elastic alloys according to the invention has muchbroader applications encompassing both medical as well as otherindustrial applications. A device of the present invention is generallyshown at 13 in FIGS. 2–3. Tool fitting 20 may be a cutting tool fitting(as shown in FIG. 1) and preferably has a cannulation 22 that is alignedwith another cannulation 7 formed in shaft 5 when device 13 is assembledaccording to the present method. The aligned cannulations 7, 20 allowdevice 13 to be placed over the top of a guide wire (not shown). Toolfitting 20 also has a radially moveable, i.e., flexible collet portion25, which is preferably an integral structure although the fitting maybe separate from the radially flexible, split portion as discussed belowin conjunction with another embodiment (FIGS. 4–6). According to thepresent method, shaft 5 is slid into collet 25. Preferably there is aninterference fit between the outer diameter D1 of shaft 5 and the innerdiameter D2 of collet 25. This interference causes collet 25 to bend outin a flower configuration as shaft 5 and fitting 20, are slid together.

Once tubular shaft 5 has been slid into collet 25, a compression sleeve10 (FIGS. 1–3) is slid over the flower shaped collet 25 and welded atjunction 12. As these components 5, 10, 25 are assembled, the collet 25is forced radially onto the shaft causing a super-elastic activation ofthe alloy forming the shaft to thereby effect a secure coupling. Thissuper-elastic reaction allows the fingers of collet 25 to contact thetube along the length L1 as shown in FIG. 3. This surface-to-surfacecontact (shown at 30 in FIG. 3) allows the device 13 to transmit torque.In essence the components are transmitting torque via friction. Thus,whenever the frictional forces are overcome by application of too muchtorque to the tool fitting 20, the fitting and shaft 5 break free of oneanother to slip rotationally. This slippage limits the amount of torquethat can be applied to the shaft. The contact surface 30 can be adjustedby design to change the length L1, in turn, adjusting the maximumapplicable torque limit to ensure that the slippage occurs before themaximum yield strength is reached in shaft 5.

Compression sleeve 10 may have an optional polymer sleeve 15pre-assembled inside. The purpose of polymer sleeve 15 is to transferstress through tool fitting 20 in a uniform manner to shaft 5 during itsuse in a bent configuration. This smoother transition ensures prematurefailure of fitting 20. Use of polymer sleeve 15 may be found necessarywhen the design of the wall thickness of tool fitting 20 becomes thin,e.g. between about 0.25 mm to 1.00 mm. Otherwise, tool fitting 20 willbe able to handle the stress eliminating the necessity for polymersleeve 15.

Another preferred embodiment of the present invention is depicted inFIGS. 4–6, particularly FIG. 4, which shows a shaft 105 made ofsuper-elastic alloy such as nitinol. As discussed relative to FIGS. 1–3,shaft 105 is similar, preferably having a cannulation 107 as shown inFIG. 6. Shaft 105 thus can exhibit the distinctive characteristic oftransferring torque while exposed to high bending forces during use.Although the exemplary uses of super-elastic alloy herein are orthopedicdrilling and reaming devices, use of this alloy in the present inventionhas much broader applications encompassing both medical as well as otherindustrial applications. The assembled device is generally shown at 113in FIGS. 5–6 and utilizes a tool fitting 120 that is modular rather thana unitary component (see collet 25 in FIGS. 1–3). Tool fitting 120 mayhave a cannulation 122, which is useful while aligned with the shaftcannulation 107 to allow it to be placed over the top of a guide wire(not shown). Tool fitting 120 is sized with inner diameter D104 and hasa modular radially flexible portion comprising a series of nitinolwashers 125 that together are collar-shaped. Washers 125 each have anouter diameter D103 and an inner diameter D102. During assemblyaccording to another, preferred method of the present invention, shaft105 having diameter D101 is slid into the tool fitting 120. Preferablyupon assembly there is an interference fit between the outer diameterD101 and the inner diameter D102 and an interference fit between theouter diameter D103 and D104. This interference causes a radialcompression in the flexible washers 125 causing surface-to-surfacecontact as shown at 130 along length L101. The radially flexible washers125 end up interposed between the shaft 105 and the tool fitting 120.This can be accomplished through various assembly methods. In oneinstance the flexible washers 125 can be pre-assembled with the toolfitting 120 and then the shaft, 105 can subsequently be introduced tothe assembly. Preferably though, the shaft 105 can be loosely placed inthe tool fitting 120 and then the washers can be advanced into the toolfitting 120. As they are slid into place they compress via interferencebetween the outer dimension D103 of the washers 125 and the innerdimension D104 of the tool fitting. This interference causes asuper-elastic reaction in the washers 125 causing their inner dimensionD102 to compress against the shaft's 105 outer dimension D101 causingthe surface-to-surface contact as shown at 130 along length L101. It isthis surface to surface contact shown at 130 in FIG. 6 that allows theassembly to transmit torque. In essence the components are transmittingtorque-using friction. If the frictional forces are overcome by applyingtoo much torque to the tool fitting 120 the components 120,105 breakfree and slip rotationally. This rotational slip limits the amount oftorque that can be applied to the shaft. If the contact surface 130 isadjusted during design by changing the length L101 the maximumapplicable torque limit can be adjusted to ensure that the slippageoccurs before the maximum yield strength is reached in the shaft 105.

An additional sleeve 110 can be added and welded at junction 112 with anoptional polymer sleeve 115 pre-assembled inside. The purpose of thepolymer sleeve is to transfer bending stress through the tool fitting120 in a uniform manner to the shaft 105 during use in a bentconfiguration. This smoother transition ensures premature failure of thefitting. The use of this polymer sleeve 115 may only be necessary whenthe design of the wall thickness of the tool fitting 120 becomes thin,somewhere on the order of 0.25 mm to 1 mm. Other wise the tool fitting120 will be able to handle the stress eliminating the necessity for theadditional polymer sleeve 115 and sleeve 110.

Although the invention has been described with reference to preferredembodiments thereof, it is evident to those of skill in the art thatvarious modifications and improvements may be made without departingfrom the spirit and scope of the invention as defined by the appendedclaims.

1. A torque-transmitting coupling assembly comprising: a) a split colletmember having an exterior surface and an opening; b) an elongated shaftmember made of an alloy selected from a group of alloys consisting ofsuper-elastic, bi-metal alloys and super-elastic, tri-metal alloys,including super-elastic, bi-metal and tri-metal nickel-titanium alloys,received within the opening; and c) a sleeve member having an aperturethat receives the exterior surface of the collet, whereupon relativemotion among at least two of the members causes the opening to contactthe shaft, the shaft and the collet being urged into surface-to-surfacecontact sufficiently to induce a martensitic activation of thesuper-elastic alloy, thus securing the members together in a fixedrelative position.
 2. The assembly of claim 1 wherein interferingengagement of the exterior surface with the an aperture compresses theopening against the shaft, inducing the martensitic activation in theshaft.
 3. The assembly of claim 1 wherein the shaft is tubular with acannulation.
 4. The assembly of claim 3 wherein either the sleeve orcollet has a cannulation aligned with the shaft cannulation, for commonpassage of a guide wire there through.
 5. The assembly of claim 1further comprising an inter-positional polymer sleeve for transmittingbending stress in the assembly.
 6. The assembly of claim 1 whereinsurface-to-surface engagement occurs along one or more contact areasthat frictionally carries the applied torque.
 7. The assembly of claim 6wherein the contact area is calibrated to slip at a preset torque beforethe failure strength of the shaft is reached.
 8. The assembly of claim 1wherein the collet is adapted for connection to a cutting tool fittingor powered instrument.
 9. The assembly of claim 1 wherein the colletfurther comprises a cutting tool fitting.
 10. The assembly of claim 9further adapted for coupling to a powered instrument.